Unit notes and practice questions

Grade 11 Physics unit 3: motion in one and two dimensions

Everything here is for unit 3 of the Grade 11 Physics textbook: what the chapter covers, the short notes, practice questions with their answers, and the lesson video where one exists.

4
Cards: 10
Practice questions on unit 3: 47–94
The textbook pages for unit 3

Short notes for unit 3

The cards filed under this chapter on the Grade 11 Physics page. Open one at a time.

Acceleration sign and speeding up or slowing downCommon mistake

When you use it

Use when a question asks whether a moving object is speeding up or slowing down, or when it gives a negative acceleration.

Watch out

A negative acceleration does not always mean slowing down. The object speeds up when velocity and acceleration have the same sign and slows down when they have different signs.

Drafted from Grade 11 Physics, pages 47-94, then checked twice before it went up

Centripetal acceleration in uniform circular motionFormula

a_c = \frac{v^2}{r}

a_c = \frac{v^2}{r}

$a_c$: centripetal acceleration, directed towards the centre $m\,s^{-2}$
$v$: tangential (linear) speed $m\,s^{-1}$
$r$: radius of the circular path $m$

When you use it

Use when an object moves in a circle at constant speed and you need the acceleration towards the centre.

Watch out

Use the radius, which is half the diameter. Convert km/h to m/s by dividing by 3.6 before substituting, for example 50.4 km/h = 14 m/s.

Drafted from Grade 11 Physics, pages 47-94, then checked twice before it went up

Kinematic equations for uniform accelerationReference tablev_f = v_i + at · s = (v_i + v_f)/2 t · s = v_i t + 1/2at^2 · +2

Equation	Missing quantity
$v_f = v_i + at$	$s$
$s = \frac{v_i + v_f}{2}\,t$	$a$
$s = v_i t + \frac{1}{2}at^2$	$v_f$
$s = v_f t - \frac{1}{2}at^2$	$v_i$
$v_f^2 = v_i^2 + 2as$	$t$

When you use it

Use for straight line motion with constant acceleration when displacement, velocity, acceleration or time is needed. Pick the equation that leaves out the quantity you do not need.

Watch out

These equations apply only when acceleration is constant. They do not apply if acceleration changes.

Drafted from Grade 11 Physics, pages 47-94, then checked twice before it went up

Velocity change under constant accelerationFormula

v_f = v_i + at

v_f = v_i + at

$v_f$: final velocity $m\,s^{-1}$
$v_i$: initial velocity $m\,s^{-1}$
$a$: constant acceleration $m\,s^{-2}$
$t$: time interval $s$

When you use it

Use when acceleration is constant and you need the final velocity after a time t.

Watch out

Velocities and acceleration are vectors. Choose one positive direction first and give each quantity its sign before substituting. Use the equation only when acceleration is constant.

Drafted from Grade 11 Physics, pages 47-94, then checked twice before it went up

Practice questions on unit 3

10 multiple-choice questions written against this chapter, each with its answer and the reason. Try one before you open the answer.

Question 1

According to kinematics, what is acceleration?

A.The rate of change of velocity with time
B.The total distance divided by total time
C.The displacement per unit time in a fixed direction
D.The product of velocity and time elapsed

Show the answer and why

Answer: A The rate of change of velocity with time

Why

Acceleration is defined as the rate of change of velocity with respect to time. It is a vector quantity measured in m/s².

Question 2

In a displacement versus time graph, what does the slope of the line represent?

A.Acceleration
B.Velocity
C.Total distance
D.Centripetal force

Show the answer and why

Answer: B Velocity

Why

The slope of a displacement-time graph is the vertical change (displacement) divided by horizontal change (time), which equals velocity.

Question 3

What is meant by the terminal velocity of a falling body?

A.The velocity when an object starts to fall from rest
B.The maximum velocity of a freely falling body where its acceleration becomes zero
C.The minimum speed required to escape the Earth gravitational field
D.The average velocity of a projectile at maximum height

Show the answer and why

Answer: B The maximum velocity of a freely falling body where its acceleration becomes zero

Why

Terminal velocity is the constant maximum velocity reached by a falling object when the upward drag force equals the downward force of gravity, so acceleration is zero.

Question 4

An object moves at a constant speed in a circular path. What is the direction of its centripetal acceleration?

A.Along the tangent in the direction of motion
B.Away from the center of the circle
C.Always towards the center of the circle
D.Opposite to the direction of velocity

Show the answer and why

Answer: C Always towards the center of the circle

Why

Centripetal acceleration is caused by the continuous change in the direction of velocity and is always directed towards the center of the circular path.

Question 5

A car in Hawassa accelerates uniformly from rest to a speed of 72 km/h (20 m/s) in 8.0 seconds. What is the magnitude of its acceleration?

A.1.25 m/s2
B.2.5 m/s2
C.5.0 m/s2
D.9.0 m/s2

Show the answer and why

Answer: B 2.5 m/s2

Why

Using the kinematic formula a = (v_f - v_i) / t: with v_i = 0, v_f = 20 m/s, and t = 8.0 s, we get a = 20 / 8.0 = 2.5 m/s².

Question 6

An Ethiopian Airlines Boeing airplane lands on a runway at Bole International Airport with an initial velocity of 40.0 m/s and decelerates uniformly at 1.50 m/s². What length of runway does it need to come to a complete stop?

A.266.7 m
B.400.0 m
C.533.3 m
D.600.0 m

Show the answer and why

Answer: C 533.3 m

Why

Using v_f² = v_i² + 2as: here v_f = 0, v_i = 40.0 m/s, and a = -1.50 m/s². Thus, 0 = (40.0)² + 2(-1.50)s, which gives s = 1600 / 3.0 = 533.3 m.

Question 7

A wheel has a diameter of 60 cm. If its angular velocity is 40 rad/s, what is the tangential velocity at the rim of the wheel?

A.6 m/s
B.12 m/s
C.24 m/s
D.120 m/s

Show the answer and why

Answer: B 12 m/s

Why

The radius is half of the diameter: r = 60 cm / 2 = 30 cm = 0.3 m. Using v_t = ωr, we get v_t = 40 rad/s × 0.3 m = 12 m/s.

Question 8

What physical quantity is represented by the area between a velocity versus time graph curve and the time axis?

A.Acceleration
B.Displacement
C.Instantaneous speed
D.Time interval

Show the answer and why

Answer: B Displacement

Why

The area under a velocity-time graph line and the time axis gives the magnitude of the displacement covered by the moving body during that time interval.

Question 9

A particle moves along a straight line with its velocity given by v(t) = t² - 2t (in m/s). What is the instantaneous acceleration of the particle at t = 2 s?

A.0 m/s2
B.2 m/s2
C.4 m/s2
D.6 m/s2

Show the answer and why

Answer: B 2 m/s2

Why

The functional form of acceleration is the time derivative of velocity: a(t) = 2t - 2. Evaluating at t = 2 s gives a(2) = 2(2) - 2 = 2 m/s².

Question 10

A ball is projected vertically upwards with an initial velocity of 20 m/s from the top of a building. Taking g = 10 m/s², how much time does it take to return to its initial release level?

A.2 s
B.4 s
C.5 s
D.8 s

Show the answer and why

Answer: B 4 s

Why

At the starting level, the vertical displacement is y = 0. Using y = v₀t - (1/2)gt², we have 0 = 20t - 5t², which factors into 5t(4 - t) = 0. Thus, the time to return is t = 4 s.

The app holds 12 questions on this unit, marked as you go. These are the first 10.

The textbook pages for unit 3

Unit 3 runs from page 47 to page 94 of the Ministry of Education edition.

Ministry of Education edition, English, 338 pages

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All Grade 11 Physics short notes

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